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At Commom rail diesel engine, rail pressure regulator is controlled based on rail pressure sensor signal in order to maintain optimum rail pressure according to current engine rpm and load.

However when the problem that leads rail pressure to out of target value intended by ECM occurs due to mechanical or electronical reason, ECM shuts engine down and sets DTC by limiting fuel (stops injector operation) in order to prevent engine from being controlled abnormally. "rail pressure monitoring error" is the DTC which diagnose 1).supplying state of low pressure fuel and 2).mechanical operating conditions of high pressure pump and 3).rail pressure regulator indirectly based on rail pressure sensor output voltage and rail pressure sensor duty. thus, repair relavant to this DTC requires mechanics the total understand of fuel system.

P1186 is set when the rail pressure is lower than the specified minmum value for each engine speed ( below 2400RPM : 120bar, above 2800RPM : 200bar). This code is due to 1) rail pressure regulator sticking at opened position and valve seat wear 2) excessive fuel leakage of injectors 3) abnormal fuel pumping quantity at high pressure pump 4) insufficient fuel supply from low pressure pump.